YANKEE DISTRICT OF THE AMERICAN ROSE SOCIETY

 

 Rose Diseases
Symptoms, Causes and Remedies

Compiled by Mike & Angie Chute 

 

At our July meeting we had a three-hour seminar that concentrated on insect and disease controls. Over eighty members and invited guests from URI Master Gardeners and New England Botanical Gardens who were in attendance received several packets of handouts. One of the handouts dealt with rose diseases, listing the most common diseases found in our area, the symptoms, causes and remedies. This information is so essential in recognizing and dealing with rose diseases that I have decided to include it in this issue. I have also compiled a table of suggested fungicides, along with the recommended dosages, as suggested by Rosemania, an on-line mail order source. --- Editor’s Note

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Black Spot

It is the most significant disease of outdoor roses on a world wide basis.  The fungus has been widely distributed with cultivated roses and no real complete resistance is believed to exist.

Symptom: Dark black and sooty spots on the leaves. The spots tend to be round, varying in size from pinpoint to quarter sized. The infected leaves turn yellow leading to leaves dropping completely from the plant.  It attacks the plant from the bottom up.

Cause: The fungus can over-winter in infected leaves and canes and moves via wind currents or splashing water.

Remedy: Remove dropped leaves and other debris and discard; Spray with fungicides known to be effective on blackspot such as Funginex, BannerMaxx, Daconil 2787 or select varieties with a known resistance.

Anthracnose

Symptom: Often confused with blackspot, it starts out as small black spots turning to purple and brown and finally light brown or tan with a red or purple margin. Spotting, yellowing and severe defoliation and shot hole can result under moist spring conditions.

Cause: Cool, moist spring conditions.

Remedy: Remove old leaves from around the base of plants and prune out canes that have infections. The same spray program that is used for blackspot should work for anthracnose.

Powdery Mildew

It is one of the most prevalent and serious diseases of roses.

Symptoms: Leaves fold at mid-rib or are distorted. White or gray powdery material appears on the forming buds and leaf tops or undersides. It attacks the plant from the top down.

Cause: Typically occurs with warm days, high humidity and cool nights.

Remedy: Pruning and removal of infected plant material is the first level of defense followed by the application of fungicides such as Funginex and Immunox.  Some gardeners may use baking soda and soap. It helps to have good air circulation and ample sunlight.

Downy Mildew

It is a common rose disease that occurs under moist conditions and is found throughout the United States. All species of cultivated and wild roses are susceptible.

Symptoms: Leaves, stems and flowers may manifest purple to red or brown irregular blotches. Dark, irregular splotches appear on the leaves. Advanced infections will have yellowing of leaves with brown necrotic areas and noticeable leaf drop.

Cause: This systemic fungus disease is present in the soil and will begin to cause problems when night temperatures reach 55 to 65 degrees Fahrenheit with 85 percent humidity.

Remedy: Cut back the defoliated plant and clean up the debris. Apply BannerMaxx or Daconil at the recommended dosage.

 

Rust

Generally not found in this area. It is far more prevalent on the west coast. Occasionally, roses infected with rust are found here due to the fact that they were shipped from the west coast with the disease. Rust cannot survive through a New England winter.

Symptoms: Orange to yellow spores appearing on the undersides of leaves.

Botyritis

Symptoms: Brown spots on petals or red-pink spots on lighter colored buds. Brown or gray fuzzy mold around bloom.

Cause: High humidity and rainy, cool periods. Often found in the autumn.

Remedy: Removal and discarding infected parts of the plant are the simplest and best remedies along with maintaining good air circulation. Spray with a fungicide.

Rose Mosaic Virus

It is transmitted by propagation and requires tissue-to-tissue contact. It cannot be spread by any other means.

Symptoms: The foliage displays a variety of yellow patterns or may not display any symptoms at all. Infected plants are slower to develop then healthy plants, produce fewer and less quality blooms with shorter life spans. Typical foliage symptoms can appear in the spring, disappear for a few weeks and may reappear again.

Cause: Tissue-to-tissue contact usually initiated at the propagation stage.

Remedy: There is no cure for mosaic virus. Purchase only quality plants that have no symptoms of the disease. If you have an infected plant, it will not spread to other plants in the garden. You may continue to grow the plant until it dies.

Rose Cankers

Canker fungus is most active during the cold time of the year when roses are not actively growing. The fungi usually cannot produce the disease when conditions are favorable for plant growth.

Symptoms: The stems will yellow, often have red spots and later become brown or black.

Cause: In spring, pruning cuts or wounds can be colonized by canker causing fungi.

Remedy: Removal of infected canes and a general spray program for fungal disease will reduce canker problems.

Crown Gall

This is the only serious bacterial disease of roses.

Symptoms: A spongy, round to irregular tumor type of tissue commonly found growing at or below the bud union.

Cause: A soil-borne bacteria which infects plants through wound sites on the roots or crown. The bacteria may live in the soil for many years.

Remedy: Inspect new plants before placing them in a garden. However, some plants may have latent infections which can become evident several years after planting. The plant may survive for years with a gall but could serve as a reservoir for the bacteria. The best remedy is removal of the plant. In some cases soil removal is necessary to remove the bacteria. In other cases new roses have been successfully planted at the site. Galls may be cut out of plants and the wounds treated with bleach or antibiotic creams such as bacitracin.

Credits: ARS Consulting Rosarian Manual; Jackson & Perkins, Root of the Problem

Fungicides

Dosage/Gallon

Effectiveness

Toxicity

Banner Maxx

1/3 to 2/3 teaspoon

14 days

II – Warning

Daconil

˝ teaspoon

1 week

II - Warning

Immunox

2 Tablespoons

1 week

II - Warning

Toxicity Category II is Moderately Toxic. You are reminded to be cautious when using spray materials. DO NOT spray without proper protective gear. Wear long pants and long sleeve shirts to cover arms and legs or use a chemical resistant suit such as Tyvek. Also protect yourself by using a respirator, goggles and chemical resistant gloves. The National Pesticide Telecommnica-ions Network’s 24 hour, toll-free number to answer                                                                                             questions on the proper use or effects of pesticides is 1- 800-858-7378.

Source of Fungicide information is Rosemania.com

 Reprinted from Rhode Island Rose Review August 2004

 

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